It is known that particularly in automobiles, so-called press-hardened components composed of sheet steel are used. These press-hardened components composed of sheet steel are high-strength components that are particularly used as safety components in the region of the vehicle body. In this connection, the use of these high-strength steel components makes it possible to reduce the material thickness relative to a normal-strength steel and thus to achieve low vehicle body weights.
In press-hardening, there are basically two different possibilities for manufacturing such components. They are divided into the so-called direct and indirect methods.
In the direct method, a sheet steel blank is heated to a temperature greater than the so-called austenitization temperature and if need be, kept at this temperature until a desired degree of austenitization is achieved. Then, this heated blank is transferred to a forming die and in this forming die, is shaped into the finished component in a one-step forming process and in so doing, by means of the cooled forming die, simultaneously cooled at a speed that is greater than the critical hardening speed. This produces the hardened component.
In the indirect method, first, possibly in a multi-step forming process, the component is formed until it is almost completely finished. This formed component is then likewise heated to a temperature greater than the austenitization temperature and if need be, kept at this temperature for a desired, necessary period of time.
Then this heated component is transferred and inserted into a forming die that already has the dimensions of the component or the final dimensions of the component, if need be taking into account the thermal expansion of the preformed component. After the closing of the in particular cooled die, the preformed component is consequently cooled in this die at a speed that is greater than the critical hardening speed and is thus hardened.
In this connection, the direct method is somewhat simpler to implement, but only permits shapes that can actually be produced by means of a one-step forming process, i.e. relatively simple profile shapes.
The indirect process is somewhat more complex, but is also able to produce more complex shapes.
In addition to the need for press-hardened components, a need has also arisen to produce such components not out of uncoated sheet steel, but rather to provide such components with a corrosion protection layer.
In the automotive field, the corrosion protection layer can be composed either of rather infrequently used aluminum or aluminum alloys or of significantly more frequently used zinc-based coatings. In this connection, zinc has the advantage that it provides not just a barrier protection layer like aluminum does, but also a cathodic corrosion protection. In addition, zinc-coated press-hardened components fit better into the overall corrosion protection concept of vehicle bodies since in the construction technique that is currently popular, they are generally galvanized as a whole. In this respect, it is possible to reduce or eliminate contact corrosion.
But both methods could involve disadvantages that have also been discussed in the prior art. In the direct method, i.e. the hot forming of press-hardened steels with zinc coatings, microcracks (10 μm to 100 μm) or even macrocracks occur in the material; the microcracks occur in the coating and the macrocracks even extend through the entire cross-section of the sheet. Components of this kind with macrocracks are unsuitable for further use.
In the indirect process, i.e. cold forming with a subsequent hardening and remaining forming, microcracks in the coating can also occur, which are also undesirable, but far less pronounced.
Thus far—except for one component produced in Asia—zinc-coated steels have not been used in the direct method, i.e. hot forming. With this method, preference is given to using steels with an aluminum/silicon coating.
An overview is given in the publication “Corrosion resistance of different metallic coatings on press hardened steels for automotive”, Arcelor Mittal Maiziere Automotive Product Research Center F-57283 Maiziere-Les-Mez. This publication states that for the hot forming process, there is an aluminized boron/manganese steel that is sold commercially under the name Usibor 1500P. In addition, steels that are pre-coated with zinc for purposes of cathodic corrosion protection are sold for the hot forming method, namely galvanized Usibor GI, which has a zinc coating containing small percentages of aluminum, and a so-called galvannealed, coated Usibor GA, which has a zinc coating containing 10% iron.
It is also noted that the zinc/iron phase diagram shows that above 782° C., there is a larger region in which liquid zinc-iron phases occur as long as the iron content is low, in particular less than 60%. But this is also the temperature range in which the austenitized steel is hot formed. It is also noted that if the forming occurs at a temperature greater than 782° C., then there is a high risk of stress corrosion due to liquid zinc, which presumably penetrates into the grain boundaries of the base steel, resulting in macrocracks in the base steel. Furthermore, at iron contents of less than 30% in the coating, the maximum temperature for the forming of a safe product without macrocracks is less than 782° C. This is the reason why direct forming methods are not used with these steels, but instead the indirect forming method is used. This is intended to bypass the above-mentioned problem.
Another possibility for bypassing this problem should lie in using galvannealed, coated steel, which is because the iron content of 10% that was already present at the beginning and the absence of a Fe2Al5 bather layer lead to a more homogeneous formation of the coating out of predominantly iron-rich phases. This results in a reduction or elimination of zinc-rich, liquid phases.
“‘STUDY OF CRACKS PROPAGATION INSIDE THE STEEL ON PRESS HARDENED STEEL ZINC BASED COATINGS’, Pascal Drillet, Raisa Grigorieva, Grégory Leuillier, Thomas Vietoris, 8th International Conference on Zinc and Zinc Alloy Coated Steel Sheet, GALVATECH 2011—Conference Proceedings, Genoa (Italy), 2011” indicates that galvanized sheets cannot be processed in the direct method.
EP 1 439 240 B1 has disclosed a method for hot forming a coated steel product; the steel material has a zinc or zinc alloy coating on the surface of the steel material and the steel base material with the coating is heated to a temperature of 700° C. to 1000° C. and hot formed; before the steel base material with the zinc or zinc alloy coating is heated, the coating has an oxide layer that is chiefly composed of zinc oxide in order to prevent the zinc from vaporizing during the heating. A special process sequence is provided for this purpose.
EP 1 642 991 B1 has disclosed a method for hot forming a steel in which a component composed of a boron/manganese steel is heated to a temperature at the Ac3 point or higher, is kept at this temperature, and then the heated steel sheet is formed into the finished component; the formed component is quenched through cooling from the forming temperature during the forming or after the forming in such a way that the cooling rate at the MS point at least corresponds to the critical cooling rate and the average cooling rate of the formed component from the MS point to 200° C. lies in the range from 25° C./s to 150° C./s.
The applicant's patent EP 1 651 789 B1 has disclosed a method for manufacturing hardened components out of sheet steel; according to this method, formed parts composed of a sheet steel that is provided with a cathodic corrosion-protection layer are cold formed and undergo a heat treatment for purposes of austenitization; before, during, or after the cold forming of the formed part, a final trimming of the formed part and required punching procedures or the production of a hole pattern are carried out and the cold forming as well as the trimming and punching and arrangement of the hole pattern on the component are carried out 0.5% to 2% smaller than the dimensions that the final hardened component should have; the formed part, which has been cold formed for the heat treatment, is then heated in contact with atmospheric oxygen in at least some regions to a temperature that permits an austenitization of the steel material and the heated component is then transferred to a die and in this die, a so-called form hardening is carried out in which the contacting and pressing (holding) of the component by the form hardening dies cause the component to be cooled and thus hardened and the cathodic corrosion protection coating is composed of a mixture of essentially zinc and additionally, one or more oxygen-affine elements. As a result, on the surface of the corrosion protection coating, an oxide skin composed of the oxygen-affine elements forms during the heating, which protects the cathodic corrosion protection layer, in particular the zinc layer. In addition, in the method, the scale reduction of the component with regard to its final geometry takes into account the thermal expansion of the component so that neither a calibration nor a forming are required during the form hardening.
The applicant's patent WO 2010/109012 A1 has disclosed a method for manufacturing partially hardened steel components in which a blank composed of a hardenable steel sheet is subjected to a temperature increase that is sufficient for a quench hardening and after a desired temperature is reached and if need be, after a desired holding time, the blank is transferred to a forming die in which the blank is formed into a component and simultaneously quench hardened or the blank is cold formed and the component resulting from the cold forming is then subjected to a temperature increase, with the temperature increase being carried out so that a component temperature that is required for a quench hardening is reached and the component is then transferred to a die in which the heated component is cooled and thus quench hardened; during the heating of the blank or component for the purpose of increasing the temperature to a temperature required for the hardening, in the regions that should have a lower hardness and/or a higher ductility, absorption masses are placed or are spaced apart from these regions by a narrow gap; the absorption masses, with regard to their expansion and thickness, their thermal conductivity, and their thermal capacity and/or with regard to their emissivity, are especially dimensioned so that the thermal energy acting on the component in the region of the component that remains ductile flows through the component into the absorption mass so that these regions remain cooler and in particular, the temperature required for hardening is not reached or is only partially reached so that these regions cannot harden or can harden only partially.
DE 10 2005 003 551 A1 has disclosed a method for hot forming and hardening a steel sheet in which a steel sheet is heated to a temperature above the Ac3 point, then undergoes a cooling to a temperature in the range from 400° C. to 600° C., and is only formed after reaching this temperature range. This reference, however, does not mention the crack problem or a coating and also does not describe a martensite formation. The object of the invention therein is the formation of intermediary structures, so-called bainite.
The object of the invention is to create a method for producing sheet steel components, which are in particular provided with a corrosion protection layer, with regions of different hardness and/or ductility while avoiding local stresses in the component, as well as distortion and cracks of the kind that can otherwise be caused by “liquid metal assisted cracking.”